1. Field of the Invention
The present invention relates to a cellular communication system, and more particularly to a receiver for extending the dynamic range of a received signal strength indicator for use in a cellular communication system.
A receiver according to the present invention for extending the dynamic range of a received signal strength indicator is based on Korean Application Serial No. 19782/1995 which is incorporated herein by reference.
2. Description of the Related Art
In general, a received signal strength indicator (RSSI) of a signal received through an antenna is measured in the reception port of a cellular communication system. The measured RSSI is used for selecting a control channel in a standby mode and is used for a hand-off function.
FIG. 1 shows the connection of a circuit in the prior art for measuring the RSSI in the reception port of the conventional cellular communication system. Referring to FIG. 1, a radio signal received through an antenna 2 is amplified by a radio frequency (RF) amplifier 4. The amplified signal is mixed by a first mixer 8 with a signal oscillated by a first local oscillator 6 to generate a first intermediate frequency (IF) signal. The first IF signal is mixed by a second mixer 12 with a signal oscillated by a second local oscillator 20 to generate a second IF signal. The second IF signal is amplified by an IF amplifier 14. The amplitude of the amplified second IF signal is limited by a limiter 16 to be less than or equal to a predetermined magnitude. Generally, the signal output from the limiter 16 is an audio signal supplied to a user.
An arithmetic amplifier 18 converts the current of a signal output from the IF amplifier 14 and limiter 16 into a direct voltage (DV) and outputs the same. It is possible to measure the RSSI indicating the strength of the signal received through the reception port of the cellular communication system using the output DV. The following Table 1 indicates exemplary DV outputs from the arithmetic amplifier 18 in correspondence with the strength of the signal input to the IF processing unit 10.
TABLE 1 ______________________________________ RSSI of IF processing unit [dBm] DV [V] ______________________________________ -120 0.5 -110 1.0 -100 1.5 -90 2.0 -80 2.5 -70 3.0 -60 3.5 -50 4.0 -40 4.5 -30 5.0 -20 5.0 -10 5.0 ______________________________________
Referring to the above Table 1, if the RSSI of the signal input to the second mixer 12 of the IF processing unit 10 is -120 dBm, the DV level output from the arithmetic amplifier 18 is 0.5 V. If the RSSI of the signal input to the second mixer 12 of the IF processing unit 10 is -100 dBm, the DV level output from the arithmetic amplifier 18 is 1.5 V. If the RSSI of the signal input to the second mixer 12 of the IF processing unit 10 is -30 dBm, the DV level output from the arithmetic amplifier 18 is 5.0 V.
FIG. 2 shows an RSSI characteristic curve of the prior art IF processing unit 10 and of the overall reception port in the cellular communication system as indicated in Table 1. Referring to Table 1, the dynamic range of the RSSI of the IF processing unit 10 having the second mixer 12, IF amplifier 14, limiter 16 and arithmetic amplifier 18 is about 90 dB (-30 dBm to -120 dBm), as shown by a solid line G1 of FIG. 2. In other words, when the RSSI of the signal input to the second mixer 12 of the IF processing unit 10 is -30 dBm to -120 dBm, the RSSI can be measured. If the RSSI of the signal input to the second mixer 12 of the IF processing unit 10 is greater than -20 dBm or less than -120 dBm, the RSSI cannot be measured. The interval in which the RSSI can be measured is called an RSSI dynamic range. The interval in which the RSSI cannot be measured is called an RSSI saturated region.
Since the gain of the RF amplifier 4 and first mixer 8 disposed in front of the IF processing unit 10 is about 20 dB to receive the band frequency (for example, 869 MHz to 894 MHz) of the cellular communication system, the RSSI dynamic range of the overall reception port of the cellular communication system shown in FIG. 1 is decreased to about 70 dB, as shown by a single dashed line G2 of FIG. 2. In other words, since the RSSI characteristic curve is moved to the RSSI saturated region by 20 dB, which is the gain of the components disposed in front of the IF processing unit 10, the RSSI dynamic range of the overall reception port of the cellular communication system corresponds to the interval of -30 dBm to -100 dBm. In other words, only when the RSSI of the signal received through the antenna 2 corresponds to the interval of -30 dBm to -100 dBm, the RSSI can be measured.
Also, even if the signal having the RSSI corresponding to the interval of -30 dBm to -100 dBm is input, the gain of the RF amplifier and first mixer should be increased to improve the reception sensitivity of the reception port of the cellular communication system. In such an implementation, the RSSI dynamic range would be much more decreased. Thus, a control channel cannot be selected in a standby mode of the reception port, nor can be allowed in using a hand-off function.